1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted on a perpendicular magnetic recording type hard disk drive (HDD) or the like and a method of manufacturing such a medium.
2. Description of the Related Art
Various information recording techniques have been developed along with recent increase of capacity in information processing. Particularly, the areal recording density of hard disk drives (HDDs) using a magnetic recording technique has continued to increase at an annual rate of about 100%. Recently, a 2.5-inch magnetic disk used for an HOD or the like has been required to have an information storage capacity greater than 200 Gbytes per disk. In order to meet such a demand, it is necessary to achieve an information recording density greater than 400 Gbits/inch2.
In order to achieve a high recording density of a magnetic recording medium used for an HDD or the like, a perpendicular magnetic recording type magnetic disk has been proposed in recent years. A perpendicular magnetic recording type medium has a magnetic recording layer with a magnetization easy axis oriented perpendicular to a surface of a substrate. As compared to a conventional in-plane magnetic recording type, the perpendicular magnetic recording type can reduce a thermal fluctuation phenomenon, in which the thermal stability of recorded signals is deteriorated due to the superparamagnetism phenomenon so that the recorded signals are lost. Accordingly, the perpendicular magnetic recording type is suitable to increase the recording density.
Use of a CoCrPt—SiO2 perpendicular magnetic recording medium has been proposed for a magnetic recording medium of a perpendicular magnetic recording type because a CoCrPt—SiO2 perpendicular magnetic recording medium exhibits high thermal stability and excellent recording characteristics (see T. Oikawa et. al., IEEE Trans. Magn, vol. 38, 1976-1978 (2002) (Non-Patent Document 1)). Size reduction of magnetic grains in a magnetic recording layer and improvement of the magnetic coercive force (Hc) are achieved by forming a granular structure in which non-magnetic grain boundaries are formed by segregating SiO2 between magnetic grains in which crystals of a hexagonal close-packed (hop) structure of Co have grown continuously into a columnar shape. It has been known that oxide is used for non-magnetic grain boundaries (non-magnetic portions between magnetic grains). For example, use of one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 has been proposed (JP-A-2006-024346 (Patent Document 1)).
Meanwhile, important factors to increase the recording density include improvement of magnetostatic characteristics, such as the magnetic coercive force (Hc) and the reversed domain nucleation magnetic field (Hn), and improvement of electromagnetic characteristics, such as the overwrite performance (OW performance), the signal-to-noise ratio (SNR), and the reduction of the track width. Among other things, improvement of the magnetic coercive force (He) and the SNR is important to read and write data accurately at a high speed even with recording bits having a small area.
Improvement of the SNR is achieved mainly by noise reduction of a magnetic transition region in a magnetic recording layer. Factors effective in noise reduction include improvement in crystal orientation of a magnetic recording layer, reduction of the grain diameter of magnetic grains, and isolation of magnetic grains. Particularly, in order to improve the crystal orientation of a magnetic recording layer, an underlayer is provided in a perpendicular magnetic recording medium. Crystal grains of a magnetic recording layer grow on crystal grains of the underlayer. Therefore, the crystal orientation of the magnetic recording layer can be improved. Because the crystal orientation of the underlayer affects the crystal orientation of the magnetic recording layer, a crystal seed layer (non-magnetic layer) is provided underneath the underlayer so as to improve the crystal orientation of the underlayer.
The seed layer (non-magnetic layer) has heretofore been formed by a single layer. Recently, a perpendicular magnetic recording medium having a seed layer of a double-layer structure has been developed (JP-A-2007-184019 (Patent Document 2)). According to Patent Document 2, a seed layer is formed of an amorphous alloy containing Cr and a crystal alloy containing Ni as a principal component. With this seed layer, a perpendicular magnetic recording medium can maintain a good SNR and can have excellent corrosion resistance.